I have a customer that has a bunch of connect amps and is wanting to airplay content. From what I understand those older units don’t support airplay. If they added 1 new Sonos amp, would they be able to group airplay content to the other zones from the new amp?

That is my understanding as well, BUT the "New" Sonos product you add has to be S1/S2 compatible and needs to be added to the existing Sonos system as an S1 device.

In my experience, AMP/Ports/S2 compatible Connects/Connect Amps, will add to an S1 system automatically if they are new out of the box, or have not been activated on S2. If the item has been activated on S2, it will need to be defaulted and downgraded.

This link might be helpful.

[Link: support.sonos.com]

Just run this into an input
[Link: amazon.com]